1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly to a method and an apparatus for transmitting a data channel and a control channel in the same transmission time interval.
2. Description of the Related Art
In general, transmission schemes employed in a Single Carrier-Frequency Division Multiple Access (SC-FDMA) system, which is an example of a wireless communication system, includes a Distributed FDMA (DFDMA) scheme and a Localized FDMA (LFDMA) scheme.
FIG. 1 is a block diagram illustrating a structure of a transmitter in a typical LFDMA system. It is possible to implement another type of transmitter as well as the transmitter shown in FIG. 1, which includes a Discrete Fourier Transform (DFT) precoder 101 and an Inverse Fast Fourier Transform (IFFT) unit 102. Implementation of the DFT precoder 101 and the IFFT unit 102 as shown in FIG. 1 facilitates changes of the LFDMA system parameters without a high hardware complexity.
A difference between the OFDM and the SC-FDMA is discussed in view of the transmitter structure. Further to the IFFT unit 102, which is used for multi-carrier transmission in a typical OFDM transmitter, the LFDMA transmitter additionally includes the DFT precoder 101 connected to the input side of the IFFT unit 102. M number of sub-carriers output from the DFT precoder 101 are mapped to the input points N−M to N−1 of the IFFT unit 102, so that they occupy and are transmitted by a band configured by adjacent sub-carriers. In general, the input/output size N of the IFFT unit 102 has a value greater than the input/output size M of the DFT precoder 101. The output signal of the IFFT unit 102 is transmitted after passing through the Parallel-to-Serial (P/A) converter 104 and the Cyclic Prefix (CP) adder 106.
In an uplink transmission, control information that should be transmitted through a control channel by a User Equipment (UE) includes Acknowledge/Non-Acknowledge (ACK/NACK) or Channel Quality Indicator (CQI) that is necessary mainly for the transmission of downlink packet data.
FIGS. 2A and 2B illustrate transmission of control information in a typical SC-FDMA system.
Referring to FIG. 2A, a separate frequency resource 201 different from that of the data channel is allocated to a control channel in order to transmit control information in an SC-FDMA system. In the case of transmitting control information through the allocated frequency resource 201, a UE cannot transmit packet data. This is because simultaneous transmission of packet data and control information in the same transmission interval cannot satisfy the single carrier characteristic, which results in an increase in the Peak to Average Ratio (PAPR).
Therefore, when it is necessary to transmit control information in a transmission interval for packet data transmission by a UE, the control information is transmitted together with data through the frequency resource 203 of the data channel as shown in FIG. 2B. In other words, packet data, control information, and a reference signal are time-multiplexed and transmitted in the same frequency resource 203.
FIG. 3 is a block diagram illustrating a structure of a transmitter for multiplexing and transmitting packet data and control information before input of them to a DFT precoder in a typical SC-FDMA system. As shown, data 301 including P number of symbols and control information 302 including S number of symbols are multiplexed into M symbols by a multiplexer 303, which are then input to a DFT precoder 304 having a size of M. As described above, outputs of the DFT precoder 304 are mapped to inputs of an IFFT 305 having a size of N.
When the packet data and the control information are multiplexed before the DFT precoder as shown in FIG. 3, it is necessary to allocate the scheduled M input symbols based on the information quantity of each of the data channel and the control channel. According to one typical scheme, such as the Wideband Code Division Multiple Access (WCDMA) scheme, input symbols are allocated in accordance with the set transport format of each of the control channel and the data channel. In other words, when the number of symbols is set according to a fixed transport format of the control channel, the other input symbols except for the symbols of the control channel are used for data transmission. This is because it is usual that a data rate of a data channel is variable according to the scheduling while the transport format of the control channel is fixedly set by higher layer signaling.
However, when the transport format of the control channel is fixed as described above, as many symbols as the symbols occupied by the control channel are reduced from the symbols transmissible through the packet data channel, so as to reduce the data rate. At this time, the data rate is reduced by the amount corresponding to [the number of symbols used for the control channel×a Modulation and Coding Scheme (MCS) level]. Therefore, when a UE has obtained a high MCS level scheduled for high speed data transmission, the number of data bits that are not transmitted may increase due to the transmission of the control information.